The existing IPv4 address shouldn't be changed/removed, as some services
still require it.

The pre 0.25 convention that requires both Local Backend and Master Backend addresses
to be the same in order for a backend to be considered a master backend still applies.
It has been expanded to allow for the Master Backend address to equal either the
Local IPv4 or IPv6 Backend address.

Some (non MythTV) programs require their IPv6
addresses enclosed in brackets ([]) or the
addition of a trailing percent sign and Zone Index
(e.g. %eth0.) Neither of these should be used
with MythTV.

Use mythtv-setup to configure any slave
backends with their own IPv6 addresses.

Restart the backend(s).

Which IPv6 address should be used?

The most likely choice is a Unique Local Address (ULA.)
Use ULA's if all access to the backend will be
within a LAN.

Note that even though ULAs are global addresses, no self
respecting router will pass them out on the internet,
much like IPv4 Private Network addresses (e.g. 192.168.1.123.)

Use a search engine and look for: 'Unique Local Address generator'
to find a site that will create ULAs correctly like
this, for example.

Be sure any firewalls have been adjusted to accept the IPv6 addresses being used.

If the MythTV configuration is correct, but no IPv6 traffic is seen during, for example recording playback, Master Backend Override may be enabled in mythtv-setup General section and the Master Backend IP address is still set to an IPv4 address.

Tools and sample output

Warning: The output of some tools and in logfiles may expose IPv6 addresses.
Some addresses contain a modified MAC address or globally addressable IPv6 addresses.
When posting output to a public form, consider camouflaging them.

Additional information is available logs for mythbackend
and mythfrontend if the --loglevel debug
option is used.

IPv6 addresses in the logs don't follow the
convention that allows consecutive 0:0s to be represented by ::.
So, the address used in this page: fd01:a:b:1::123
will appear as fd01:a:b:1:0:0:0:123.
This is not a problem, only a convention used by the underlying
Qt code.

These examples are all from a host with mythbackend
running and IPv6 configured. MySQL is running and using IPv4.
Obviously, these tools can be run on frontends and slave backends.

To make sure mythbackend and mysqld
are listening for connections:

sudo netstat -Wpant | egrep '3306|6543'

When the backend is running, but no slave backends/frontends are connected, expect: